What is called a broadcast communication and what is called a point-to-point communication are used as a dedicated short range communication (DSRC) between a road side apparatus serving as a base station and an in-vehicle apparatus serving as a mobile station (cf. JP-4296127B and JP-2010-9280A). The broadcast communication is a communication between a road side apparatus and multiple unspecified vehicles traveling on a road. An example of the broadcast communication is a VICS (Vehicle Information and Communication System: registered trade mark). The point-to-point communication is a communication between a road side apparatus and a specific vehicle traveling on a road. An example of the point-to-point communication is an ETC (Electronic Toll Collection System). In recent years, a hybrid communication is becoming popular. The hybrid communication is a communication in which the broadcast communication and the point-to-point communication are simultaneously preformed.
In relation to the hybrid communication, the inventors of the present application have discovered the following problem, which will be now explained below as a part of the present disclosure.
According to Non-Patent Document 1, in a case of no response from the in-vehicle apparatus to the road side apparatus's assignment of an activation channel (ACTC) within a predetermined time, an association (initialization) procedure including re-selection of a communication frequency is repeated. According to Non-Patent Document 2, in performing the broadcast communication, the in-vehicle apparatus starts a communication connection management timer (CTO) in response to receiving the broadcast communication, and then updates the CTO each time the in-vehicle apparatus receives the broadcast communication. Then, if the CTO times out after the in-vehicle apparatus departs from a communication area of the corresponding base station and becomes unable to communicate with the base station, it is determined that the vehicle departs from the communication area. According to the above standard, furthermore, in the case of the hybrid communication including the point-to-point communication and the broadcast communication, the point-to-point communication or the broadcast communication, whichever received first, is given to priority.
Non-Patent Document 1: Dedicated Short Range Communication System, ARIB Standard, ARIB STD-T75, Version 1.5, December 2008, Association of Radio Industries and Business (ARIB)
Non-Patent Document 2: DSRC Application Sub-layer, ARIB Standard, ARIB STD-T88, Version 1.1, December 2007, Association of Radio Industries and Business (ARIB)
According to the above standard, the broadcast communication is first performed and the point-to-point communication is subsequently performed in the hybrid communication. In this case, if transmission reception level between the road side apparatus and the in-vehicle apparatus is stable in a communication area (i.e., a vehicle is in a stable communication area 101, see FIG. 4), a problem does not arise in the communication. This is because the broadcast communication and the point-to-point communication are sequentially started in the hybrid communication. However, if the transmission reception level between the road side apparatus and the in-vehicle apparatus is disturbed and unstable (i.e., a vehicle is in an unstable communication area 102), the in-vehicle apparatus cannot transmit the communication to the road side apparatus although the in-vehicle apparatus can receive the communication from the road side apparatus in some cases (see FIG. 4). As illustrated in FIG. 4, because of reflection or interference of a radio wave transmitted from a road side apparatus 11, an unstable communication area 102 is formed distant from the road side apparatus 11 than a stable communication area 101 is from the road side apparatus 11. In a normal case, a vehicle 103 equipped with an in-vehicle apparatus passes through this unstable communication area 102 in a short time, and a major problem does not arise in communication establishment.
However, if the vehicle 103 stays in the unstable communication area 102 a predetermined time period or more because of, for example, traffic jam or the like, the point-to-point communication times out. This is because the point-to-point communication is not performable in the unstable communication area 102 although the broadcast communication, which is transmitted from the road side apparatus, is performable. As a result, when the vehicle 103 enters into the stable communication area 101 after passing through the unstable communication area 102, there is a possibility that the point-to-point communication has already timed out; thus, there arises a problem of being unable to perform the point-to-point communication in the hybrid communication.